This invention generally relates to equipment for use in handling and storing gas, such as gas produced by decomposing organic liquid-solid mixtures, and more particularly to a gas storing or digester apparatus which includes a tank for storing a large volume of liquid and a cover having a cylindrical sidewall that extends into the liquid so that a liquid seal is formed near the bottom edge of the sidewall, thereby trapping a volume of gas between the liquid in the tank and the cover to buoy up the cover.
U.S. Pat. No. 3,288,295 to E. M. Kelly discloses an anaerobic digester of the type including a tank and a dome-shaped cover that includes a buoyant chamber or trough formed as an integral part of the cover itself. Gas-evolving material (hereinafter called sludge) to be anaerobically digested is admitted into the tank through a sludge feed pipe, while supernatant or treated material is withdrawn through another conduit located at the desired surface level of the liquid. The size and configuration of the buoyant chamber is accurately predetermined having regard to the weight of entire cover assembly and the specific gravity of the sludge to be treated.
The Kelly patent further shows corbels incorporated in the concrete sidewall of the tank of the digester. The corbels are provided to limit downward travel of the cover and support during periods of clean-out. The patent further shows guide rollers located at the periphery of the cover to facilitate vertical travel. The rollers maintain the spacing of the cover with respect to the tank wall. The Kelly patent further discloses a plurality of guide strips rigidly mounted to the sidewall of the tank to mate with complementary channel strips affixed to the sidewall of the digester cover.
Although the arrangement for supporting and guiding the cover as disclosed in the Kelly patent has been generally satisfactory, it is subject to a major shortcoming. Occasionally, the cover strikes the corbels with sufficient force to break the corbels from the sidewall. This presents a major problem to the operator of the digester because the digester must be evacuated and the digester cover completely removed to permit entry into the tank so that the corbels may be repaired. The repair is particularly difficult due to the method of construction of the corbels. Another problem is that the corbels are not visible during operation, so it is impossible to ascertain any minor damage to the corbels.